1. Field of the Invention
The present invention relates to a manufacturing method for an optical element such as a diffraction element, which is provided with minute grooves.
2. Description of Related Art
A diffraction element which is used in an optical head device to record onto or reproduce from a CD, a CD-R or a DVD is provided with many fine or minute grooves formed at a prescribed pitch on the surface of a base member of an optical element. The minute grooves cannot be formed by a conventional working machine with a degree of working accuracy of 0.1 μm. Therefore, the diffraction element is conventionally manufactured by using semiconductor processes.
However, the manufacturing costs are high when the semiconductor processes are used as the manufacturing method for the optical element. Further, in order to form the minute grooves having a protruded part and a recessed part by the semiconductor processes, a mask layer is formed by utilizing a photo lithography technique and etchings are performed on the base member through apertures of the mask layer. Using these techniques, slant faces cannot be formed. Accordingly, in recent years, it has been considered to form the minute grooves on a diffraction element by using an ultra-precision working machine whose resolution power is 1 nm.
However, even though the ultra-precision working machine is used, when the groove is machined, a high degree of precision is not attained for the inclination of the bottom part of the recessed part or the upper part of the protruded part, which form the minute grooves. The degree of precision of the inclination is ±1° at most. The low precision causes the variation of the height differences between right and left side faces of the groove. As a result, the diffracted beams with a first or more order, which are to be diffracted symmetrically, will not have equal diffracted efficiency unfavorably. For example, the diffracted efficiencies of a (+) first-order diffracted beam and a (−) first-order diffracted beam are not equal to each other.
In other words, in the conventional machine working, all errors, that is, an error caused by the machining precision of the working machine itself, an error caused by the configuration of a tool, a mounting error of the tool to the working machine, an error caused by the configuration of the base member to be machined, and a mounting error of the base member to the working machine, are summed up and cause inclination errors of the bottom surface of a recessed part or the upper surface of a protruded part which forms the groove.